1. Field of Invention
The present invention relates to organic semiconductors and to the use thereof in organic electronic devices.
2. Description of the Prior Art
Organic semiconductors are being developed for a number of different applications which can be ascribed to the electronics industry in the broadest sense. The structure of organic electroluminescent devices (OLEDs) in which these organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. No. 4,539,507, U.S. Pat. No. 5,151,629, EP 0676461 and WO 98/27136. However, these devices still exhibit considerable problems which require urgent improvement:    1. The compounds usually used do not have a sufficiently low LUMO (lowest unoccupied molecular orbital). Compounds having a relatively low LUMO are required for easier electron injection and thus for a reduction in the operating voltage.    2. In systems in accordance with the prior art, one or more dopants are usually used in a host material in the emitting layer. It would be sensible to have compounds which can be used as pure substance in the emitting layer since this represents a technical simplification in device production.    3. The lifetime of the organic electroluminescent device is still inadequate for long-lived, high-quality applications.    4. The thermal stability of many organic compounds currently used in organic electroluminescent devices is inadequate, meaning that considerable problems arise both in the purification of the material by bulk sublimation and also during application of the material by thermal evaporation. This applies, in particular, to compounds which contain styrylamino groups, as are frequently used as blue-emitting compounds.
For fluorescent OLEDs, principally condensed aromatic compounds, in particular anthracene or pyrene derivatives, are used in accordance with the prior art as host materials, especially for blue-emitting electroluminescent devices, for example 9,10-bis(2-naphthyl)anthracene (U.S. Pat. No. 5,935,721). WO 03/095445 and CN 1362464 disclose 9,10-bis(1-naphthyl)anthracene derivatives for use in OLEDs. Further anthracene derivatives are disclosed in WO 011076323, in WO 01/021729, in WO 04/013073, in WO 04/018588, in WO 03/087023 or in WO 04/018587. Host materials based on aryl-substituted pyrenes and chrysenes are disclosed in WO 04/016575. For high-quality applications, it is necessary to have available improved host materials.
As prior art in blue-emitting compounds, mention may be made of the use of arylvinylamines (for example WO 04/013073, WO 04/016575, WO 04/018587). However, these compounds are thermally unstable and cannot be evaporated without decomposition, which requires high technical complexity for the synthesis and OLED production and thus represents a technical disadvantage. A further disadvantage is the emission colour of these compounds: whereas dark-blue emission (CIE y coordinates in the range 0.15-0.18) is described in the prior art with these compounds, it has not been possible to reproduce these colour coordinates in simple devices in accordance with the prior art. By contrast, green-blue emission is obtained here. It is not evident how truly blue emission can be produced using these compounds. For high-quality applications, it is therefore necessary to have available improved emitters, particularly in relation to device and sublimation stability and emission colour.
In phosphorescent OLEDs, the matrix material used is frequently 4,4′-bis-(N-carbazolyl)biphenyl (CBP). The disadvantages are short lifetimes of the devices produced therewith and frequently high operating voltages, which result in low power efficiencies. In addition, CBP has an inadequately high glass transition temperature. Furthermore, it has been found that CBP is unsuitable for blue-emitting electroluminescent devices, which results in poor efficiency. In addition, the structure of the devices comprising CBP is complex since a hole-blocking layer and an electron-transport layer additionally have to be used. Improved triplet matrix materials based on keto compounds are described in WO 04/093207. However, toxic inorganic cyanides are disclosed for the synthesis of the best of the matrix materials described therein, and consequently the preparation of these materials is ecologically unacceptable. The glass transition temperature of other matrix materials described therein is still inadequate.
The electron-transport compound used in organic electroluminescent devices is usually AlQ3 (aluminium trishydroxyquinolinate) (U.S. Pat. No. 4,539,507).
This has a number of disadvantages: it cannot be vapour-deposited without leaving a residue, since it partially decomposes at the sublimation temperature, which represents a major problem, in particular for production plants. A further disadvantage is the high hygroscopicity of AlQ3, as is the low electron mobility, which results in higher voltages and thus in lower power efficiency. In order to prevent short-circuits in the display, it is desirable to increase the layer thickness; this is not possible with AlQ3 owing to the low charge-carrier mobility and the resultant increase in voltage. Furthermore, the inherent colour of AlQ3 (yellow in the solid), which can result in colour shifts, especially in blue OLEDs, due to reabsorption and weak re-emission, has proven very unfavourable. Blue OLEDs can only be produced here with considerable reductions in efficiency or colour location. In spite of the said disadvantages, AlQ3 to date still represents the best compromise for the various requirements of an electron-transport material in OLEDs.
Thus, there continues to be a demand for improved materials, in particular host materials for blue-fluorescent emitters and triplet emitters, but also emitting compounds, in particular blue-emitting compounds, hole-transport materials and electron-transport materials, which are thermally stable, result in good efficiencies and at the same time in long lifetimes in organic electronic devices, give reproducible results in the production and operation of the device, are readily accessible synthetically and in high yields and have high thermal stability.